Carbon nanotubes have been applied in various fields including memory devices, electronic amplifiers, gas sensors, electronic wave shielding apparatuses, electrode plates of electrochemical storage apparatuses (e.g., secondary battery, fuel battery and supercapacitor), field emission displays, and polymer composites, due to their good mechanical, chemical and electrical properties.
In general, a carbon nanotube has a diameter of a few to several tens nm and a length of several tens μm, and it may be of an anisotropic configuration such as a single-wall, multi-wall or rope shape. Further, most carbon nanotubes have chiral configurations helically arranged along the tubular axis, and such chiral characteristics of the carbon nanotubes represent an important factor for determining the electrical conductivity of a particular batch of carbon nanotubes.
However, the chirality of carbon nanotubes produced by conventional methods such as are discharging, laser ablation, and chemical vapor deposition, is difficult to control, and, therefore, the carbon nanotubes produced by such methods are in the form of a mixture of the two types: conductive and semi-conductive. Accordingly, in order to obtain carbon nanotubes having suitable conductivity characteristics for its final use, it is required to selectively separate one desired type of carbon nanotubes from a mixture of two types of carbon nanotubes.
As such an attempt, there has been developed a method of employing a surfactant such as octadecylamine (ODA) to separate only the metallic type carbon nanotubes from the mixture-type carbon nanotubes (see Journal of the American Chemical Society, 125, p 3370, 2003). However, this method has the problems of difficulty in removing the surfactant remaining after the separation and a low yield. Further, a method of separating metallic type carbon nanotubes from a suspension of carbon nanotubes in D2O by electrophoresis was attempted (see Science, 301, p 345, 2003), but this method gives a low productivity and a low yield.
Therefore, it is required to develop an effective method for separating a desired type carbon nanotube in a large scale.